Selective Lateral Lithiation of Methyl BODIPYs: Synthesis

Letter pubs.acs.org/OrgLett

Selective Lateral Lithiation of Methyl BODIPYs: Synthesis, Photophysics, and Electrochemistry of New Meso Derivatives Eduardo Palao,† Santiago de la Moya,*,† Antonia R. Agarrabeitia,† Ixone Esnal,‡ Jorge Bañuelos,‡ Iń ̃igo López-Arbeloa,‡ and María J. Ortiz*,† †

Departamento de Química Orgánica I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, Ciudad Universitaria s/n, 28040 Madrid, Spain ‡ Departamento de Química Física, Universidad del Pais Vasco-EHU, Apartado 644, 48080 Bilbao, Spain S Supporting Information *

ABSTRACT: Selected meso BODIPYs (chemically reactive, difficult to obtain by established procedures, or photophysically or electrochemically attractive) have been obtained by unprecedented selective lateral lithiation of 8-methylBODIPYs. The physical study of the obtained new meso BODIPYs reveals interesting tunable properties related to the activation of intramolecular charge-transfer processes, endorsing the new synthetic methodology as useful for the development of smarter BODIPY dyes for technological applications. methyl (Scheme 1),10b which demonstrates the higher enaminelike reactivity of that methyl when compared with those located

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes are a well-known family of fluorophores with significant use in lasing, sensing, bioimage, photodynamic therapy, or optoelectronics, among other valuable photonic applications.1 This is due to three main factors: (1) excellent photophysical properties coming from the BODIPY chromophore, (2) high solubility in organic solvents improving processability steps required for the development of specific dyed materials (i.e., organic films), and (3) a core that can be selectively functionalized, allowing the fine modulation of its photophysical signature.1−5 In relation with the latter, some of the most successful BODIPY transformations involve selective electrophilic aromatic substitution (generally halogenations at C2/6, but also at other positions),2 nucleophilic aromatic substitution (mainly of halogen at C3/5),3 nucleophile substitution at boron (mainly to prepare C- and O-BODIPYs),4 as well as metal-catalyzed C−C cross couplings in different positions (mainly involving haloBODIPYs and palladium).5 C8-functionalized (meso) BODIPYs constitute an important subclass within the BODIPY family due to their success in certain photonic applications (e.g., molecular probing or ratiometric sensing by fluorescence signaling).6 Some functionalities (e.g., bromoalkyl or chloroalkyl) can be introduced in the BODIPY meso position when its core is being constructed (i.e., before boron complex formation). 7 However, interesting meso BODIPYs for specific photophysical applications cannot be easily achieved by this way.8 This fact has promoted a special interest in the development of synthetic procedures to meso BODIPYs from more accessible BODIPY precursors.9 Along these lines, it is known that 8-methylBODIPYs undergo Knoevenagel-like reaction with aldehydes to yield the corresponding meso-alkenyl derivatives.10 Interestingly, it is also known that 3,5,8-trimethylBODIPY and 1,3,5,7,8-pentamethylBODIPY undergo the reaction regioselectively at the meso © 2014 American Chemical Society

Scheme 1. Knoevenagel-like Reactivity of Selected 8MethylBODIPYs

at C3/5 or C1/7. These results prompted us to investigate the selective enamine-like chemistry of 8-methylBODIPYs, aimed at expanding the available synthetic methods to valuable meso BODIPYs. Our first objective was to explore the acylation of 8methylBODIPYs, since the expected 8-(2-oxoethyl)BODIPYs are unknown and potentially interesting due to the well-known chemistry and photophysics of the carbonyl group. For this purpose, commercial 1a was used as starting meso-methyl BODIPY and highly reactive methyl chloroformate as acylating reagent (Scheme 2). Different common bases and reaction conditions were explored (see Scheme 2; note that five methyl groups are potentially reactive). Treatment of 1a with piperidine, DMAP, or NaH in different solvents and conditions and then addition of methyl chloroformate resulted in the recovery of 1a (Scheme 2). On the other hand, the use of t-BuOK or BuLi led to the destruction of the BODIPY framework. In the case of t-BuOK, Onucleophilic attack on the boron center, and then boron Received: May 26, 2014 Published: August 25, 2014 4364

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Scheme 2. Approaches for Methoxycarbonylation at Meso Methyl in 1a (See the Supporting Information for More Details)

4a (Scheme 3). More interesting is the related reaction of mesononylated 3b, since α-functionalization of the meso group is now possible (Scheme 3). However, the decrease of the acidity, and also the increase of the sterical hindrance, of the nonyl C8methylene explains the observed formation of 4b. Trying to find appropriate conditions to achieve the mesomethyl halogenation of 1c, we observed that the use of N,Ndimethylformamide/I2 (DMF/I2) surprisingly led to the formation of 2j (see Scheme 3). It must be noted that DMF alone does not react as electrophile in the used reaction conditions (see the Supporting Information, SI). The same reactivity was tested for 1a and 1b (products 2k and 2l, respectively, in Scheme 3). Although the observed reactivity is striking (unprecedented up to our knowledge), it should be noted that all the obtained dimethylamino derivatives were isolated with good yields (52−65%, see Scheme 3). This fact discards the possible initial formation of the corresponding iodoBODIPY (e.g., 2i), giving place to the observed final product by reaction with dimethylamine as a trace reagent in the reaction media. Nonetheless, the presence of dimethylamine has been also discarded (see the SI). On the other hand, the observed dimethylamination does not work when DMF/Br2 is used instead of DMF/I2 (1a and 1b were tested). In this case, mesomethyl bromination takes place exclusively (probably due to the higher electrophilic character of bromine). Moreover, no reaction was observed when N,N-diisopropylformamide (DIPF) was used instead of DMF. These results suggest that iminium cation 5 (formed by the electrophilic attack of iodine on the formamide carbonyl) is involved as the N-electrophilic reagent quenching the lithiated BODIPY (Scheme 4; see the SI for experimental evidence supporting the proposed mechanism).

decomplexation during the final hydrolysis step, should explain the observed result (the corresponding unstable dipyrromethene was detected by NMR).11 For BuLi, C-nucleophilic attack at C8, giving place to an undetected difluoroboradipyrromethane salt, is a highly plausible explanation for the observed BODIPY destruction.12 Satisfactorily, the use of LDA resulted in the formation of 2a (Scheme 2), which constitutes the first example of BODIPY chemistry based on its selective lateral lithiation.13 To study the scope of this procedure, different BODIPYs and electrophiles (E) were essayed (Scheme 3). Scheme 3. Selective Lithiation−Functionalization of 1 and 3 (See the Supporting Information for Details)

Scheme 4. Reactivity of Laterally Lithiated BODIPYs with DMF/I2 (Circles Highlight Steric Hindrance)

The initially tested BODIPYs were the commercially available PM567, PM597, and PM546 (1a−c, respectively), while the initially chosen E (molecular halogens, and acyl, sulfonyl, and sulfenyl chlorides) were selected on the basis of the potential chemical,14 electrochemical,15 or photophysical interest16 of the expected reaction products. All the conducted reactions took place selectively at the meso position (see 2a−i in Scheme 3), with the exception of the halogenation reaction of 1c, which gave place to the corresponding 2,6-dihaloBODIPY2a (iodination was tested). This result is explained by the appropriate location of unsubstituted BODIPY positions (CH) with respect to the lithiated methyl,17 joined to the known BODIPY ability to undergo electrophilic aromatic substitution at C2/C6.2 It must be noted nonetheless that, in the rest of the essayed cases, neither functionalization at a different methyl nor polyfunctionalization was detected. Expectedly, the absence of meso-methyl entails the selective lithiation of methyl group at C3/C5, yielding the corresponding functionalized BODIPY after electrophilic treatment. Thus, the methoxycarbonylation of meso-mesitylated 3a selectively yielded

The “inverted” reactivity of 5 (N- instead of C-electrophile) is explained by a double-regioselective control exerted by both reactive partners. Thus, the bulky iodine of 5 blocks its carbon center against the nucleophilic attack of an, also hindered, lithiated BODIPY, which results in the preferential reaction of the nitrogen center (Scheme 4). Thus, shielding the nitrogen of 5, by changing its methyl groups by isopropyls (DIPF instead of DMF), results in the observed lack of reactivity. The photophysics of the new meso BODIPYs was found to be ruled by the electron-donating/withdrawing properties of the group located at the meso methyl and the BODIPY core itself. Thus, the strong n electron-donating dimethylamino group of 2j−l, albeit separated from the BODIPY core by a methylene unit, promotes a drastic fluorescence quenching in these derivatives when compared to the corresponding parent (unsubstituted) BODIPYs (e.g., see 2k vs 1a, and 2l vs 1b in Table 1; see also Table S1 and Figures S1−S3, SI). This result can be explained by the activation of an intramolecular charge4365

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Table 1. Photophysical Properties of meso-BODIPYs 2a,b and 2k,l and Parent BODIPYs 1a,ba,b 1a 2a 2k 1b 2b 2l

λab (nm)

εmax (M−1cm−1)

λfl (nm)

ϕ

τ1 (ns)

ΔνSt (cm−1)

518.5 547.0 536.0 525.0 554.0 541.5

83000 63000 57000 78000 57000 66000

534.0 563.0 547.0 566.5 586.0 602.0

0.79 0.71 0.05 0.33 0.11 0.03

5.63 6.13 0.24c 4.15 1.29c 0.39c

560 520 375 1395 985 1855

at the C8-methyl, is promoted when the electron-donating ability of the BODIPY is high enough (as it occurs in 2b by the presence of the tert-butyl groups); otherwise, the process is thermodynamically nonviable (cf. 2a vs 2b in Figure 1), allowing the detection of an efficient fluorescence signal. The photophysical effect caused by the halogenation at the meso methyl in 1b, to give 2g or 2h, is noteworthy. The photophysics of 1b are controlled by the steric hindrance exerted by the tert-butyls, which distorts the chromophore planarity, mainly in the excited state leading to large Stokes shifts (Table 1). However, halogenated 2g and 2h exhibit low Stokes shifts (see Table S1, SI), suggesting lower differential distortion upon excitation. On the other hand, the higher fluorescence efficiency of iodinated 2h when compared with brominated 2g (24% vs 9%, see Table S1, SI) suggests lower ICT ability for the former due to the less electronegative character of iodine. The noticeable influence of the meso-methyl groups on modulating the photophysics of the corresponding nonsubstituted parent BODIPY is also observed in the modulation of the electrochemistry (Figure 2 and Figures S4−S6, SI). Thus,

a Tetrahydrofuran solution. bFor full detailed data, see Table S1 (Supporting Information). cMain lifetime from biexponential fits.

transfer (ICT) process. Indeed, the fluorescence efficiency was recovered after protonation (2j was tested).6b,g Conversely, the introduction of electron-withdrawing methoxycarbonyl at the meso methyl in 1a and 1c, to give 2a and 2c, respectively, retains the fluorescence ability of the parent BODIPY (cf. 2a vs 1a in Table 1 and in Figure S1, SI; cf. 2c vs 1c in Table S1 and Figure S2, SI). The same fact occurs when methoxycarbonyl is introduced at the C5-methyl instead of the meso methyl (see 4a in Table S1, SI). However, locating the same group at the meso methyl in di-tert-butylated 1b, to give 2b, quenches the fluorescence (cf. 2b vs 1b in Table 1 and in Figure S3, SI). The last effect is also observed by locating the same group at the C5-methyl in 1b (see 4b in Table SI, SI). Interestingly, it was observed that all the derivatives of di-tertbutylated 1b bearing electron-withdrawing groups at the meso methyl (2b−f) are characterized by a poor fluorescence response as well, mainly in polar media (see Table S1, SI). This 1b effect must be caused by the increased electron-donating ability of the BODIPY core, by the branched tert-butyl groups, which switches on an ICT process from the BODIPY core to the electronwithdrawing group located at meso methyl. Note the higher inductive effect (+I) of tert-butyl (1b derivatives) when compared with ethyl (1a derivatives) or hydrogen (1c derivatives). The absence of red-shifted emission bands or new absorption bands for these 1b derivatives suggests that the involved ICT state is not fluorescent, and it is not directly populated, but through to the locally excited (LE) state (see 2b in Figure 1). This nonfluorescent de-excitation of the LE state should be favored in polar media, as it is observed (see Table S1, SI) due to the higher stabilization of the polar ICT state. It is important to highlight here that the population of the ICT state, from the BODIPY core to the electron-withdrawing group

Figure 2. Cyclic voltammograms of 1b (black), 2l (blue), 2b (red), and 2e (green) in acetonitrile (see the Supporting Information for details).

the introduction of the n electron-donating dimethylamino group alters the oxidation wave of the parent BODIPY (two irreversible oxidation peaks appear in the anodic region), whereas the reversible shape of the cathodic wave is almost the same (e.g., 2l vs 1b in Figure 2). The lower oxidation potential of the dimethylaminated derivatives agrees with the ICT process (BODIPY-core reductive) proposed for these compounds. Moreover, their oxidation and reduction waves are now closer, suggesting lower energy gap by HOMO-state destabilization, in agreement also with their spectral red-shifts when compared to the corresponding parent (e.g., see 2l vs 1b in Table S1 and also in Figure S3, SI). Oppositely, electron-withdrawing groups (e.g., methoxycarbonyl or halogen) alter the reduction ability of the parent nonsubstituted BODIPY: irreversible reduction peaks appear at smaller potentials in the cathodic region, whereas the oxidation ability remains almost unchanged7d (e.g., see 2b and 2e vs 1b in Figure 2). The easier reduction also explains the red shifts observed in these meso BODIPYs (electron-withdrawing substituted) when compared to their parents (LUMO-state stabilization). In summary, a new synthetic method to achieve meso BODIPYs is described. This method involves the selective lateral lithiation of 8-methylBODIPY precursors, followed by electrophilic capture of the formed azaenolate-like organolithium. The procedure works well for different electrophiles, with the exception of highly reactive Br2 or I2 when reacting with nonentirely substituted BODIPY substrates (polyhalogenation takes place). Interestingly, DMF/I2 is demonstrated to cause the

Figure 1. Absorption (bold) and fluorescence (dashed) spectra of 2a (black) and 2b (red) in tetrahydrofuran (see the Supporting Information for details) and energy diagram explaining differential fluorescence behavior by ICT process (radiative processes in green, nonradiative in red). 4366

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Funct. Mater. 2013, 23, 4195. (c) G. Durán-Sampedro, G.; Esnal, I.; Agarrabeitia, A. R.; Bañuelos, J.; Cerdán, L.; García-Moreno, I.; Costela, A.; López Arbeloa, I.; Ortiz, M. J. Chem.Eur. J. 2014, 20, 2646. (d) Sánchez-Carnerero, E. M.; Moreno, F.; Maroto, B. L.; Agarrabeitia, A. R.; Ortiz, M. J.; Vo, B. G.; Muller, G.; de la Moya, S. J. Am. Chem. Soc. 2014, 136, 3346. (5) For example, see: (a) Rohand, T.; Qin, W.; Boens, N.; Dehaen, W. Eur. J. Org. Chem. 2006, 4658. (b) Yin, S.; Leen, V.; Snick, S. V.; Boens, N.; Dehaen, W. Chem. Commun. 2010, 46, 6329. (c) Ortiz, M. J.; GarciaMoreno, I.; Agarrabeitia, A. R.; Duran-Sampedro, G.; Costela, A.; Sastre, R.; Lopez Arbeloa, F.; Bañuelos Prieto, J.; Lopez Arbeloa, I. Phys. Chem. Chem. Phys. 2010, 12, 7804. (d) Lakshmi, V.; Ravikanth, M. Dyes Pigments 2013, 96, 665. (e) Rihn, S.; Erden, M.; De Nicola, A.; Retailleu, P.; Ziessel, R. Org. Lett. 2011, 13, 1916. (f) Duran-Sampedro, G.; Palao, E.; Agarrabeitia, A. R.; de la Moya, S.; Boens, N.; Ortiz, M. J. RSC Adv. 2014, 4, 19210. (6) For example, see: (a) Zeng, L.; Miller, E. W.; Pralle, A.; Isacoff, E. Y.; Chang, C. J. J. Am. Chem. Soc. 2006, 128, 10. (b) Tian, M.; Peng, X.; Feng, F.; Meng, S.; Fan, J.; Sun, S. Dyes Pigments 2009, 81, 58. (c) Rosenthal, J.; Lippard, S. J. J. Am. Chem. Soc. 2010, 132, 5536. (d) Li, P.; Xie, T.; Duan, X.; Yu, F.; Wang, X.; Tang, B. Chem.Eur. J. 2010, 16, 1834. (e) Li, P.; Fang, L.; Zhou, H.; Zhang, W.; Wang, X.; Li, N.; Zhong, H.; Tang, B. Chem.Eur. J. 2011, 17, 10520. (f) Royzen, M.; Wilson, J. J.; Lippard, S. J. J. Inorg. Biochem. 2013, 118, 162. (g) Oshikawa, Y.; Ojida, A. Chem. Commun. 2013, 49, 11373. (7) For example, see: (a) Kálai, T.; Hideg, K. Tetrahedron 2006, 62, 10352. (b) Guliyev, R.; Buyukcakir, O.; Sozmen, F.; Bozdemir, O. A. Tetrahedron Lett. 2009, 50, 5139. (c) Wang, D.; Fan, J.; Gao, X.; Wang, B.; Sun, S.; Peng, X. J. Org. Chem. 2009, 74, 7675. (d) Krumova, K.; Cosa, G. J. Am. Chem. Soc. 2010, 132, 17560. (e) Genovese, D.; Bonacchi, S.; Juris, R.; Montalti, M.; Prodi, L.; Rampazzo, E.; Zaccheroni, N. Angew. Chem., Int. Ed. 2013, 52, 5965. (f) Xie, R.; Yi, Y.; He, Y.; Liu, X.; Liu, Z.-X. Tetrahedron 2013, 69, 8541. (g) Heisig, F.; Gollos, S.; Freudenthal, S. J.; El-Tayeb, A.; Iqbal, J.; Müller, C. E. J. Fluoresc. 2014, 24, 213. (8) For example, see: Flores-Rizo, J. O.; Esnal, I.; Osorio-Martínez, C. A.; Gómez-Durán, C. F. A.; Bañuelos, J.; López Arbeloa, I.; Panell, K. H.; Metta-Magaña, A. J.; Peña-Cabrera, E. J. Org. Chem. 2013, 78, 5867. (9) For example, see: (a) Peña-Cabrera, E.; Aguilar-Aguilar, A.; González-Domínguez, M.; Lager, E.; Zamudio-Vázquez, R.; GodoyVargas, J.; Villanueva-García, F. Org. Lett. 2007, 9, 3985. (b) OsorioMartínez, C. A.; Urías-Benavides, A.; Gómez-Durán, C. F. A.; Bañuelos, J.; Esnal, I.; López Arbeloa, I.; Peña-Cabrera, E. J. Org. Chem. 2012, 77, 5434. (c) Leen, V.; Yuan, P.; Wang, L.; Boens, N.; Dehaen, W. Org. Lett. 2012, 14, 6150. (d) Wang, J.; Vicente, M. G. H.; Fronczek, F. R.; Smith, K. M. Chem.Eur. J. 2014, 20, 5064. (10) (a) Shivran, N.; Mula, S.; Ghanty, T. K.; Chattopadhyay, S. Org. Lett. 2011, 13, 5870. (b) Palao, E.; Agarrabeitia, A. R.; Bañuelos-Prieto, J.; Arbeloa Lopez, T.; Lopez-Arbeloa, I.; Armesto, D.; Ortiz, M. J. Org. Lett. 2013, 15, 4454. (11) Smithen, D. A.; Baker, A. E. G.; Offman, M.; Crawford, S. M.; Cameron, S.; Thompson, A. J. Org. Chem. 2012, 77, 3439. (12) Crawford, S. M.; Thompson, A. Org. Lett. 2010, 12, 1424. (13) Clark, R. D.; Jahangir, A. Org. React. 1995, 47, 1. (14) As example, 8-(iodomethyl)BODIPYs are valuable key intermediates to interesting dyes by nucleophilic substitution: (a) Kálai, T.; Hideg, K. Tetrahedron 2006, 62, 10352. (b) Guliyev, R.; Buyukcakir, O.; Sozmen, F.; Bozdemir, O. A. Tetrahedron Lett. 2009, 50, 5139. On the other hand, note the chemical possibilities of the rich carbonyl chemistry. (15) As an example, for the red-ox properties of related oxygenated BODIPYs, see ref 7d. (16) The lack of procedures to date for obtaining a broad series of meso BODIPYs has avoided the study on the influence exerted by certain functional groups located at the meso position (e.g., carbonyl) on the BODIPY photophysics. (17) Mathieu, J.; Gros, P.; Fort, Y. Chem. Commun. 2000, 951.

dimethylamination of the studied meso-methy-lithiated BODIPYs. Lateral lithiation−functionalization of BODIPYs can also take place selectively in 3-methylBODIPYs, but in this case the absence of meso methyl is necessary. The physical study of the new meso BODIPYs demonstrates that their photophysical and electrochemical properties can be finely tuned by key structural modifications. The obtained compounds range from highly fluorescent dyes to almost nonemitting systems by the promotion of ICT, which, in turn, makes them valuable for sensing purposes. Finally, we are convinced that the reported new BODIPY reactivity, based on its selective lateral lithiation, will be highly useful for the future development of smarter BODIPYs for technological applications.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental procedures, NMR spectra, photophysical and electrochemical data, and experimental evidence supporting the mechanism in Scheme 4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the MINECO (MAT2010-20646-C0402 and -04) of Spain is gratefully acknowledged. I.E. thanks Gobierno Vasco for a research contract (IT339-10).

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REFERENCES

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